This invention relates to wideband antennas and more particularly to a method and apparatus for lowering the low frequency cut-off of meander line loaded antennas.
Meander line loaded antennas are described in U.S. Pat. No. 5,790,080 issued to John T. Apostolos on Aug. 4, 1998 and incorporated herein by reference. The purpose of the meander line is to increase the effective length of the antenna such that compact antennas may be designed for use for instance in cellular phones where the real estate for the antenna is limited.
With the decrease in size of wireless handsets, it is only with difficultly that one can design an antenna which will fit within the margins of the case of the wireless handset and still be usuable in dual or trimode phones which span the 830 MHz and the 1.7 and 1.9 Mz bands. Now that GPS receivers are sometimes included in wireless handsets it is important that the antenna also be able to receive the GPS frequency of 1.575 GHz.
As illustrated in U.S. Pat. No. 6,323,814 issued to John T. Apostolos on Nov. 27, 2001 and incorporated herein by reference, an improvement over Apostolos"" original patent includes a wideband version in which the meander line loaded antenna has a wide instantaneous bandwidth. In this particular antenna the feed to the antenna is through a meander line coupled between the signal source and a plannar conductor extending orthogonally from the ground plane for the antenna. This configuration offers an instantaneous bandwidth of 7:1 and has been implemented in a so called quadrature arrangement in which there are two pairs of meander line antennas arranged in opposition. The opposed pairs are orthogonally arranged to enable circular polarization.
As described in this latter, patent, the meander line is connected in series between a signal source and a plannar top conductor which is spaced from the ground plane such that the signal from the meander line is directly connected to the top plate. The result for such a feed for the meander line loaded antenna is that the low frequency cut-off of the antenna is determined by the fact that the meander line loaded antenna reactance with a shorted meander line is positive at the lower frequencies, which when added to the meander line and distributed capacity reactance results in a high VSWR at frequencies, in one embodiment, below 860 megahertz, thus limiting its usefulness in the cellular band which is centered around 830 megahertz. It is noted that in this type of antenna the drive is fed through the meander line and then to the top plate. Moreover, a quadrature arrangement is possible with this meander line design and is desirable when the antenna is mounted to the roof of a truck cab because of the circular polanization provided by the quadrature design.
Rather than having the meander line connected in series with the feed for the top plate, in the subject invention the feed from the signal source is spaced from one end of the top conductor thus to provide a capacitive feed. In this case, the meander line runs from this capacitive feed point parallel to underside of the top plate where it is folded back and then is connected by a transmission line to the underlying ground plane. The result is the shifting of the low frequency cut-off by more than 20% over the direct feed wide bandwidth meander line loaded antenna described above.
The reason for the shifting of the low frequency cut-off is the fact that the meander line loaded antenna reactance with a shorted meander line goes negative at lower frequencies. This reactance is subtracted from the meander line and distributed capacitive reactance such that at the peak of the meander line and distributed capacitive reactance waveform the reactance at the peak is cancelled by the negative going meander line antenna reactance.
With cancelled reactance at the lower frequencies, the VSWR of the antenna is decreased so that the antenna is now able to operate approximately 20-30% lower in frequency than the direct feed meander line loaded antenna described in U.S. Pat. No. 6,323,814.
In order to provide such an antenna, a horizontal top plate is coupled to the ground plane plate through a slow wave meander line and a transmission line. Signals are coupled to and from the horizontal plate through a low impedance capacitive feed. The meander line is a so-called slow meander line which has one or more loops and is connected to the top plate along the edge at which the capacitive coupling exists. The capacitive feed, in one embodiment, includes a vertical plate having an edge which is parallel to and adjacent to one edge of the top plate, with a gap existing therebetween. The slow meander line as described in the above patent is one in which signals travel down it at speeds less than the speed of light due to the differing impedances in the line.
A cap may be attached to the top plate to extend over the capacitive gap and downwardly to increase capacitance between the vertical plate and the top plate at lower frequencies.
Mounting the antenna on a finite ground plate conductor generates currents in the conductor which enhance loop mode antenna radiation at low frequencies relative to antenna dimensions to provide a volumetrically efficient antenna suitable for cell phone applications. Also the low frequency cut-off is lowered due to the meander line running from the capacitive feed point of the antenna to the ground plane. Thus there is a series connection to ground through the meander line, or opposed to having a series connection through the meander line to the signal source as was the case in the prior direct connection design.
While the subject capacitively fed meander line loaded antenna is discussed in connection with its use in cell phone applications involving not only the cellular phone band of 830 megahertz but also the PCS bands of 1.7-1.9 gigerhertz, such a capacitance fed meander line antenna also has application in a variety of different antennas designed for wideband applications.
One such application is a both vertically and circularly polarized antenna to be mounted on trucks, with the circular polarization generated by a quad arrangement of elements. The application is for a combined GPS, PCS and cellular antenna which performs both a wireless communication function with its vertical polarization and acts to receive GPS satellite signals with its circular polarization.
It has thus been found that the capacitive feed along with the loading technique described above extends the low frequency capability of the meander line loaded antenna, thus to effectively increase its already wide bandwidth.
In summary, a wideband meander line loaded antenna is provided with a capacitive feed to lower the reactance of the meander line antenna such that at lower frequencies the antenna reactance goes negative to cancel out the reactance of the meander line and distributed capacitance, the resultant lowering of the low frequency cut-off for the antenna permiting the antenna to be used, for instance, in cellular phone applications in which not only are the cellular frequencies accommodated by the antenna, but also PCS and GPS frequencies as well. With the capacitive feed the low frequency cut-off is lowered by as much as 30% over standard meander line loaded antennas.